Somatic cells have been reprogrammed into the pluripotent state by nuclear transfer (Gurdon, J. B. et al., Nature 182:64-65 (1958); Eggan, K. et al., Nature) 428:44-49 (2004), Noggle, S. et al, Nature 478:70-75 (2011)), cell fusion (Tada, M. et al., Curr Biol 11:1553-1558 (2001); Cowan, C. A. et al., Science 309:1369-1373 (2005); Blau, H. M. et al., Semin Cell Dev Biol 10:267-272 (1999)), and forced expression of transcription factors (Takahashi, K. et al., Cell 131:861-872 (2007); Chen, M. J. et al., Cell Stem Cell 9:541-552 (2011)). Somatic cells have also been reprogrammed into terminally differentiated cells such as myoblasts (Davis, R. L. et al, Cell 51:987-1000 (1987)), macrophage-like cells (Xie, H. et al., Cell 117:663-676 (2004)), beta-cells (Zhou, Q. et al., Nature 455:627-632 (2008)), hepatocyte-like cells (Sekiya, S. et al., Nature 475:390-393 (2011)), neurons (Vierbuchen, T. et al., Nature 463:1035-1041 (2010)) and endothelial cells (Ginsberg, M. et al., Cell 151:559-575 (2012)). A number of groups recently reported direct reprogramming of fibroblasts into neural stem cells/multi-lineage neural progenitors (Han, D. W. et al., Cell Stem Cell 10:465-472 (2012); Lujan, E. et al., Proc Natl Acad Sci USA 109:2527-2532 (2012); Thier, M. et al., Cell Stem Cell 10:473-479 (2012)). However, direct conversion of the somatic cells into functional engraftable multi-lineage hematopoietic stem and progenitor cells (HSPCs) has been difficult to achieve (Szabo, E. et al. Nature 468:521-526 (2010); Chambers, S. M. et al., Cell 145:827-830 (2011); Pereira, C. F. et al., Cell Stem Cell 13:205-218 (2013)).
During murine development, definitive hematopoietic stem cells (HSCs) originate in the dorsal aorta within the aorta-gonad-mesonephros (AGM) region (North, T. E. et al., Immunity 16:661-672 (2002); de Bruijn, M. F. et al., EMBO J 192:465-2474 (2000); Medvinsky, A. et al., Cell 86:897-906 (1996)). In vertebrates, including zebra fish, murine, and possibly human, HSCs are believed to emerge from the layer of hemogenic vascular cells lining the dorsal aorta floor and umbilical arteries (Zovein, A. C. et al., Cell Stem Cell 3:625-636 (2008); Boisset, J. C. et al., Nature 464:116-120 (2010); Bertrand, J. Y. et al., Nature 464:108-111 (2010); Kissa, K. et al., Nature 464:112-115 (2010)). This process depends on the expression of transcription factor (TF) RUNX1 (Chen, M. J. et al., Nature 457:887-891 (2009)). Close association of developing endothelial cells (ECs) and HSPCs in the conceptus has led to an EC-hematopoietic transition theory of hematopoiesis (Zovein, A. C. et al., Cell Stem Cell 3:625-636 (2008)).
Although it is known that HSCs and definitive erythroid/myeloid progenitors (EMPs) arise from multiple sites containing hemogenic ECs, it has been difficult to characterize the molecular programs driving the spontaneous ontogenetic transition of primitive hemogenic ECs to hematopoietic progenitors (Chen, M. J. et al., Nature 457:887-891 (2009); North, T. E. et al., Cell 137:736-748 (2009)) because the identity of key molecules and the sequence of their activity remains elusive (Orkin, S. H. et al., Cell 132:631-644 (2008)). Differential expression of TFs in hemogenic ECs progeny is linked to the early developmental decision to yield definitive HSPCs or ECs (Chen, M. J. et al. Cell Stem Cell 9:541-552 (2011)) However, it is not clear whether TFs direct these cellular fate decisions or simply promote predetermined programs in the hemogenic ECs. Microenvironmental cues provided by anatomically distinct niches—such as those within the AGM, fetal liver and placenta—are also required for physiologic expansion of primitive HSCs and effective hematopoietic development (Gekas, C. et al., Dev Cell 8:365-375 (2005)).
Modern methods of treatment of blood disorders rely on transplantation of healthy HSPCs. Currently, there are two major methods of producing a sufficient number of allogeneic and autologous HSPCs, both of which have limitations: (1) ex-vivo expansion of HSPCs (e.g. HSPCs from cord blood); and (2) directed differentiation of pluripotent cells into HSPCs. Ex-vivo expansion of healthy HSPCs is limited by donor availability and complicated by purification methods in the case of autologous transplant and HLA matching in the case of allogeneic transplantation. Directed differentiation of pluripotent cells is limited by our understanding of hematopoietic system development as well as generation of stable ECs, and is yet to yield sufficient quantities of adult transplantable HSPCs.